Tempest is thriving, but its dungeon is failing to attract serious challengers. As Rimuru searches for a solution, Diablo makes a surprising request while a powerful human family sets its sights on Tempest.
The episode 20 of That Time I Got Reincarnated as a Slime Season 4 starts with Tempest enjoying the fruits of its rapid growth. What began as a small goblin village has developed into a major city with ambitious construction projects, including a dungeon, a new parliament building, and plans for a Magic Train network that could eventually connect Tempest with Eurazania and other countries. Rimuru is pleased with how smoothly the city is developing, especially because his larger goal is to create a world where humans and monsters can coexist and prosper together.
Rimuru himself is unusually visible around town. He moves through Tempest checking on construction and daily operations, receiving friendly greetings from residents and even getting freshly made food from local merchants. His position as the leader of Tempest has clearly changed the city’s relationship with him. Rather than being treated as a distant ruler, Rimuru is now part of everyday life, with citizens happily approaching him and offering their products.
The city’s development is not limited to buildings. The Magic Train project is progressing, with a potential station for Eurazania already selected and connected to the first floor of a new castle under construction. Meanwhile, the parliament building is nearing completion, and work continues on Tempest’s increasingly elaborate dungeon. The scale of these projects shows just how quickly Rimuru’s nation has moved from a monster settlement into an organized power with infrastructure designed for international connections.
However, the dungeon has a serious problem. Challengers are repeatedly getting defeated before they can even reach the second floor. Some frustrated adventurers accuse the dungeon of being a scam because they are paying admission fees without obtaining rare equipment or making meaningful progress. The complaints have become frequent enough that Rimuru realizes the issue could eventually hurt Tempest’s wider operations.
The situation is particularly ironic because the dungeon was designed as one of Tempest’s major attractions. Instead of giving visitors an exciting challenge that encourages them to keep coming back, it is overwhelming inexperienced adventurers almost immediately. Rimuru therefore decides that something has to change. The dungeon needs to be difficult enough to remain exciting, but accessible enough that ordinary challengers can understand how to progress.
Elsewhere, the younger students are doing much better with their education. Hinata has been teaching them about spirits, an area where she has considerable expertise. The children have also settled comfortably into the school in Tempest and have quickly become part of their new class. Their happiness reinforces another important development: Rimuru no longer trusts the Kingdom of Engrassia to remain responsible for them because he has become convinced that Yuki is guilty.
The episode then shifts to a lighter moment involving Rimuru’s secretive search around the city. He encounters a situation involving an unusual food experiment and later reunites with Shuna, who has been looking everywhere for him. Shuna wants Rimuru to be the first person to taste her latest creation, and the result is another reminder of how much Tempest’s culture has evolved. Food, commerce, education, and entertainment have all become part of the nation’s expanding identity.
The political situation outside Tempest, however, is much more serious. In the Kingdom of Engrassia, the people connected to the Guild Headquarters are worried that Rimuru has become suspicious of them. Their previous dealings with the merchant from the East and their connection to Damrada have left enough circumstantial evidence for Rimuru to question their intentions. They admit that their actions may have been too aggressive, particularly because the conflict surrounding the Seven Days Clergy has drawn attention to their activities.
Despite this, they decide not to abandon their plans. Instead, they intend to avoid giving Rimuru a reason to become openly hostile. Their strategy is based on changing the situation around Tempest rather than confronting the Demon Lord directly. When Tempest’s dungeon management team reaches out to the guild and asks for help promoting the dungeon and its reward system, the proposal immediately raises suspicion. It could be a trap, but the group also recognizes that cooperating could benefit their own ambitions.
Back in Tempest, the dungeon’s problems become the focus of an operations meeting on the 100th underground floor. The management team confirms that the dungeon’s clearing rate is disastrously low. Beginners are becoming lost on the first floor, getting defeated by even weak monsters and sometimes abandoning their attempts because they cannot obtain enough food. If nothing changes, the majority of challengers may never come close to the deeper floors.
The solution comes from an unexpected source: Masayuki. He suggests creating a strategy course that functions much like a video-game tutorial. Instead of throwing inexperienced adventurers straight into the dungeon, the course would allow them to practice combat and traps in a controlled environment. The idea immediately makes sense to the management team because it gives newcomers a way to learn the basic mechanics before facing the real challenge.
Masayuki continues improving the concept by suggesting practical features that would make the dungeon more appealing. Inns and restaurants could be placed throughout the floors, while restrooms would make long expeditions more convenient. He even suggests disposable save points and item drops from defeated monsters, turning the dungeon into something closer to a full-fledged adventure game. His ideas impress the team so much that he is effectively hired on the spot.
Rimuru then introduces Masayuki to two of the most intimidating figures in Tempest. First comes Veldora, Rimuru’s best friend and the guardian of the dungeon’s 100th floor. Masayuki is understandably stunned to discover that the legendary Storm Dragon is directly involved in dungeon operations. Rimuru also introduces Ramiris, the fairy responsible for creating the dungeon and one of the members of the Octagram. The introductions leave Masayuki visibly overwhelmed, especially when Ramiris immediately suggests making him her subordinate.
With Masayuki’s ideas providing a way to fix the dungeon’s beginner problem, Rimuru takes the next major step. A new promotional campaign is announced, offering extraordinary rewards to anyone capable of reaching the deeper levels. Clearing the 50th floor earns a monthly bonus of one stellar gold coin, while defeating boss monsters provides additional rewards. The ultimate prize is even more outrageous: anyone who reaches the very bottom of the dungeon can receive 100 stellars.
The announcement immediately changes the atmosphere surrounding Tempest. Instead of being known as a dungeon where adventurers repeatedly fail on the first floor, it becomes a destination with enormous financial incentives. Adventurers begin considering a trip to Tempest, while the people managing the operation understand that the reward campaign can drive both dungeon traffic and economic activity throughout the city.
There is, of course, one enormous catch: Veldora is waiting at the bottom. The Storm Dragon proudly declares that nobody can possibly defeat him, making the 100-stellar reward almost impossible to claim. Rimuru and the others are well aware of this, but that is exactly what makes the challenge so attractive. The dungeon now has a final obstacle powerful enough to give its deepest level a legendary reputation.
Rimuru also realizes that Masayuki can play a major role in attracting adventurers. His reputation as a Hero gives the dungeon promotional value that ordinary advertising cannot match. The plan is to make Team Lightspeed a major part of the dungeon’s public image, turning Masayuki’s presence into another reason for adventurers to visit Tempest. The result is a clever combination of entertainment, competition, and economic strategy.
The strategy starts working almost immediately. As more people become interested in the dungeon, businesses in Tempest benefit as well. Tavern sales begin rising, confirming Rimuru’s belief that a successful dungeon can stimulate the entire city rather than simply generate admission fees. The dungeon is becoming an economic engine, bringing adventurers into Tempest and encouraging them to spend money while they are there.
A funny scene follows when Rimuru notices that his tea tastes different. The culprit turns out to be Shion, whose cooking experiments remain unpredictable. Yet Rimuru finds the result surprisingly delicious and praises her effort, giving Shion an enormous confidence boost. The moment adds some humor to the otherwise business-heavy episode while showing Rimuru’s willingness to encourage his followers even when their methods are unconventional.
The mood changes when Rimuru speaks with Diablo. Rimuru realizes that he never properly rewarded Diablo for his role in dealing with Falmuth. Diablo insists that serving Rimuru is reward enough, but he then makes an intriguing request. He wants Rimuru to prepare a physical vessel, similar to the doll used for Beretta, because Diablo intends to recruit an old acquaintance. This means Diablo will temporarily leave Rimuru’s side to pursue that recruitment.
Diablo’s priorities are clear. He has no interest in abandoning Rimuru; instead, he wants another subordinate capable of handling lesser matters so that he can remain devoted to Rimuru personally. His casual attitude toward destroying human nations also highlights just how dangerous Diablo remains. Rimuru agrees to prepare the vessel but warns him to stay safe during the journey.
Before the episode ends, Rimuru also checks on Duke Muese. Souei reports that there has been no unusual movement around the duke yet, but Rimuru orders him to keep watching and identify whoever is truly behind the situation. This keeps the political mystery alive even as Tempest appears to be enjoying a peaceful period of expansion.
The final scene reveals that the danger facing Tempest is far from over. In the Kingdom of Seltrosso, members of the Rosso family discuss their long-term ambition of dominating the Western nations. They view Tempest as a terrifying obstacle standing between them and their goal. An older member of the family places his hopes on Mariabell, declaring that the future of their plan depends on her.
Their objective is clear: they believe humans, rather than a Demon Lord such as Rimuru, should ultimately control the human world. The revelation gives the episode a much darker ending after its otherwise upbeat focus on Tempest’s development.
That Time I Got Reincarnated as a Slime Season 4 Episode 20 is a strong world-building episode that shows how far Tempest has come while quietly setting up its next major conflict. Rimuru’s nation is expanding through the Magic Train, parliament, dungeon, and growing commercial activity, but the dungeon’s poor clearing rate forces him and his allies to rethink how it works. Masayuki becomes surprisingly valuable when his gaming experience inspires a tutorial-style strategy course, restaurants, inns, save points, and monster item drops. The massive reward campaign then gives adventurers a powerful reason to challenge the dungeon, with Veldora waiting at the bottom as the ultimate obstacle. The episode also gives Diablo an intriguing new mission involving an old acquaintance, while Rimuru continues investigating suspicious activity surrounding Duke Muese. The biggest shift comes in the final scene, where the Rosso family makes its ambitions clear and identifies Mariabell as their hope for controlling the human world. Overall, Episode 20 balances comedy, nation-building, dungeon development, and political tension effectively, ending with the promise that Tempest’s peaceful new era may soon face a serious challenge.
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